A Mobile Station (MS) operating in a communication system such as a Code Division Multiple Access (CDMA) system attempts to establish a signaling link with a base station also known as cell or cell site by transmitting a random access message over an access channel. The access times vary and are random due to the use of random access protocols such as the ALOHA protocol or other such random access protocols. This randomness of access creates a non-deterministic access time.
In present day communication systems, such as CDMA and Wideband CDMA (WCDMA) that includes Universal Mobile Telephone Service (UMTS), MSs can support differentiating services. Services may vary based on the Quality of Service (QoS) they request from their system. For example, there are presently four QoS defined in the UMTS standard: streaming, conversational, interactive and background. As more and more types of applications are added, some of these applications may require other different types of access priority. For instance, MSs that are capable of a voice dispatch mode of operation cannot afford to wait a long time for a channel to be granted to them. While other applications like Internet browsing can absorb a longer setup time, since the MS user can wait for the system to setup before commencing to web browse.
In a CDMA or WCDMA system, when a MS attempts to transmit a random access message to the system, it calculates its transmission power level based on an open loop power control algorithm. Since the MS is not yet communicating with the system in a closed loop power control mode, it resorts to measuring its closeness to its cell site in order to calculate its initial power level setting. The open loop estimate is typically based on the signal strength of the pilot signal it receives from the cell site. On average, the MS will resort to a low transmission power level setting due to the fact that the MSs operating in the system are continually cell relocating while in idle mode in an effort to stay connected to the “best” cell site.
In a third-generation wireless communication system like a UMTS cellular system, a Random Access Channel (RACH) preamble is transmitted by an MS to the cell site in order to access the system. The RACH is an uplink-shared channel used for initial channel access to the system (e.g., for call origination, registration messages, and paging response) as well as for short data bursts (e.g., for sending Short Messaging Service packets). At the cell site, the received signal is correlated to the RACH preamble sequence in order to detect an MS access call.
Referring now to FIG. 1, there is shown a prior art RACH open loop power control procedure where an MS is attempting to gain channel access. The chart shows the uplink (from MS to cell site) activities for the MS while in the access mode. The MS in this illustrated example commences transmitting a first RACH preamble 102 at a first power level. If no acknowledgment is received from the system, a second random access preamble 104 is transmitted at a higher power level, for example 1 decibel (dB) higher than attempt 102. Again if no acknowledgment is received, the MS continues repeating the procedure of increasing the power level of its transmissions with each subsequent preamble transmission 106–114. The preamble transmission is increased until a certain predetermined power level. The MS will continue repeating the transmission of RACH preambles either until the system responds or a timer expires. If the timer expires, the MS waits a random period of time before retrying. When all retires have been used up, the MS quits attempting. In this example, after preamble 114 is transmitted, a system acknowledgment 116 occurs.
As shown in the example of FIG. 1, the system has taken a great amount of time to respond to the access request from the MS. This is of course is not acceptable for some types of QoS or applications like dispatch calls where the MS cannot wait too long to have access to the system. Given these problems, a need exists for a method and apparatus that can improve the access time for higher priority services/applications or particular users without any modification to the system's infrastructure, thus avoiding expensive system redesigns.